A device to device (D2D) discovery process is a process of determining whether a D2D-enabled user equipment (UE) is in proximity of other D2D-enabled UE. A discovering D2D-enabled UE determines whether other D2D-enabled UE is of interest to the discovering D2D-enabled UE based on the D2D discovery process. The other D2D-enabled UE is of interest to the discovering D2D-enabled UE if proximity of the other D2D-enabled UE needs to be known by one or more authorized applications on the discovering D2D-enabled UE. For example, a social networking application may be enabled to use a D2D discovery feature. The D2D discovery process enables a D2D-enabled UE of a given user of a social networking application to discover D2D-enabled UEs of friends of the given user of the social networking application, or to be discoverable by the D2D-enabled UEs of the friends of the given user of the social networking application. In another example, the D2D discovery process may enable the D2D-enabled UE of a given user of a search application to discover stores/restaurants, and the like of interest of the D2D-enabled UE of the given user of the search application in proximity of the D2D-enabled UE of the given user of the search application.
A radio spectrum or radio frequency used for D2D communication is identical to a radio spectrum or radio frequency used for general communication between a UE and base station (BS). For example, in a frequency division duplex (FDD) system, a D2D transmission and a transmission from a UE to a BS may be configured on an uplink (UL) frequency. Radio resources for a D2D transmission are allocated or reserved on the UL frequency. In a time division duplex (TDD) system, radio resources, i.e., UL radio frames/sub-frames or specific frequency resources in radio frame/sub-frames are reserved for a D2D transmission.
One of issues according to coexistence of a D2D transmission and a transmission from a UE to a BS in UL resources is collision between a hybrid automatic repeat request (HARQ) transmission by a UE and a D2D transmission by another UE.
For communication, a wireless link is divided into time slots, i.e., radio frames. Length of each radio frame is 10 ms. Each radio frame is further divided into 10 sub-frames, and length of each sub-frame is lms. A sub-frame is a minimum unit for transmitting a data packet, and is called a transmit time interval (TTI).
Meanwhile, a HARQ scheme is used for transmitting data packets from a UE to a BS. A HARQ packet carrying a data packet is transmitted in a TTI by a UE to a BS in UL, and the UE waits for feedback information from the BS. If the feedback information indicates that the HARQ packet is not successfully received, the UE retransmits the HARQ packet to the BS. If the feedback information indicates that the HARQ packet is successfully received, the UE does not retransmit the HARQ packet.
Meanwhile, a maximum retransmission number for an arbitrary HARQ packet is predetermined. In a UL, a synchronous HARQ scheme is used, and the synchronous HARQ scheme indicates that a TTI for retransmissions is fixed with respect to initial transmission (i.e., a TTI for a retransmission is at fixed timer interval from the first transmission). One HARQ process constitutes one HARQ packet transmission (including retransmissions and feedback thereof). There is a fixed time interval between two HARQ transmissions of an HARQ process. A UL HARQ scheme may be adaptive or non-adaptive. Here, an adaptive HARQ scheme indicates that resources and a modulation and coding scheme (MCS) level for HARQ retransmissions may be changed in a TTI based on signaling by a BS. The TTI in the adaptive HARQ scheme is not changed. A non-adaptive HARQ scheme indicates that all HARQ retransmissions are performed using a resource and an MCS level which are identical to a resource and an MCS level used in initial (first) HARQ transmission.
For supporting a D2D scheme in a UL frequency, certain radio sub-frames are allocated for the D2D scheme in a periodic basis. A UL HARQ packet of a HARQ process which is started in a non-D2D sub-frame for UE-BS communication by a UE may collide with D2D transmissions by another UE in a D2D sub-frame, and this will be described with reference to FIG. 1. Here, the UE-BS communication denotes communication between a UE and a BS, and the non-D2D sub-frame denotes a sub-frame which is not a D2D sub-frame. For example, the D2D sub-frame is a D2D discovery sub-frame.
FIG. 1 schematically illustrates a collision between a HARQ transmission by a UE and a D2D transmission by another UE in a communication system supporting a D2D scheme according to the related art.
Referring to FIG. 1, a radio sub-frame x is allocated/reserved for a D2D scheme. HARQ packets of a HARQ process which is started in radio sub-frames x−n*(retx_interval+1) will collide with D2D transmissions in a D2D sub-frame x. Here, n denotes an integer which is from 1 to N, and N denotes a maximum retransmission number for a HARQ packet. Referring to FIG. 1, a maximum transmission number for a specific packet is 4, and a maximum retransmission number is 3. Referring to FIG. 1, a retx_interval denotes a time interval between two HARQ packets of a HARQ process, i.e., a retransmission interval. The collision between a D2D transmission by a UE and HARQ packet transmissions by another UE affects performance of both D2D communication and UE-BS communication.
In some prior arts, all sub-frames at an interval equal to a retx_interval which is started from a predetermined sub-frame are reserved for D2D communication. The D2D communication is performed in sub-frames reserved for the D2D communication, this is very inefficient from the perspective of the D2D communication, because this will lead to large amount of resource reservation for the D2D communication. For example, for a retx_interval equal to 8 sub-frames, up to 15% of radio resources are reserved for D2D communication, and this percentage is very high. High percentage of resources which are reserved for D2D communication will lead to frequent UE wake up. Generally, in D2D communication, a plurality of sub-frames (for example, 32 sub-frames or 64 sub-frames) will be reserved at a regular interval in order of seconds to minimize a wake up number for UEs and resources for the D2D communication. Like this, resource minimization may be very important factor for enhancing performance of a communication system since a D2D communication coexists with UE-BS communication.
Therefore, there is a need for an apparatus and a method for collision avoidance between a HARQ transmission of a HARQ process by a UE and a D2D transmission by another UE in a communication system supporting a D2D scheme.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.